1. Field of the Invention
The present invention relates to a crystal pulling apparatus for obtaining a single crystal of a compound semiconductor material containing a volatile component as one constituent thereof such as gallium arsenide (GaAs), gallium phosphide (GaP), or indium phosphide (InP).
2. Description of the Prior Art
GaAs, GaP and InP melt at 1238.degree., 1470.degree. and 1050.degree. C. and have dissociation pressures as high as 0.9, 30 and 15.5 atm at such melting points, respectively. Therefore, when pulling single crystals of these compound semiconductors according to the Czochralski method, a technique has been developed in which the upper surface of the feed melt is covered with a liquid sealing agent such as, for example, B.sub.2 O.sub.3 for preventing the evaporation of arsenic (As) or phosphorous (P), and the feed melt is contained in an inert gas atmosphere of an extremely high pressure, namely, from several tens to several hundreds of atmospheres.
The feed melt usually is prepared by re-melting a polycrystalline material, but the method of preparing a feed melt directly from elemental components in the same production apparatus and subsequently pulling a single crystal is more time-saving and, so, has often been tried. In such instances, since the volatile components As and P have a vapor pressure which increases exponentially with increasing temperature, an inert gas atmosphere of a higher pressure is sometimes necessary in order to prevent the evaporation of such components.
As is well appreciated, when pulling a single crystal, it is necessary to rotate and vertically move the crucible which contains the feed melt and also to rotate and vertically move the single crystal pulling shaft which has a seed crystal attached to its lower or melt-contacting end. In conducting such processes and in the use of such apparatus for pulling up single crystals of such compound semiconductors, a major technical problem that must be dealt with and solved is how to rotate and vertically move smoothly a crucible supporting shaft and a single crystal pulling shaft without impairing the gas-tightness (hermetic seal) of the high pressure furnace in which the process is being carried out.
Further, since the liquid level lowers as the single crystal pulling operation progresses, it becomes impossible to see the interior of the crucible. It is necessary therefore, to provide a device for measuring the weight of either the single crystal being pulled or the crucible with residual feed melt in order to know to what extent the crystal pulling operation has advanced. In the past the technical difficulties of how to provide a measuring device and how to incorporate same in the crystal pulling apparatus have not been adequately solved.
The use of drive means utilizing a magnetic force for transmitting a driving force from the exterior of an electric furnace which requires gas-tightness is generally well known in the art. However, such known drive means have been all developed in association with low pressure systems. Also, such known drive means provide for either rotational movement or vertical (axial) movement, but not both. A need still exists for an effective means utilizing a magnetic force for transmitting to a single shaft located in an electric furnace (a high pressure vessel which is held in a state of an extremely high pressure from several tens to several hundreds of atmospheres) both rotating and vertically moving drive forces.
It is a primary object of the present invention to provide apparatus for pulling a single crystal of a compound semiconductor which includes a means for transmitting both a rotational drive force and a vertical movement drive force to both a crucible supporting shaft and a single crystal pulling shaft positioned within a high pressure vessel (an electric furnace held at an extremely high pressure) without impairing the gas-tightness (hermetic seal) of the pressure vessel.
It is another object of the present invention to provide apparatus for pulling a single crystal of a compound semiconductor which includes drive means for rotating and vertically moving a shaft which are arranged extremely efficiently in an enclosed cylinder of extremely small diameter.
It is a further object of the present invention to provide apparatus for pulling a single crystal of a compound semiconductor which includes effective means for measuring the weight of the single crystal or the crucible and residual feed melt without impairing at all the gas-tightness (hermetic seal) of the furnace which is being held at high pressure.
The above and other objects, features and advantages of the invention will become clear from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.